What if the shopkeeper had money to spare, and the six francs would have just been collected under his mattress otherwise?

Obviously, you don't know most shopkeepers. {grin}

Money is merely "frozen labor". Money sitting under a mattress is a potential future purchase, no different than money in a wallet or purse. Although, in some senses, it's worse because idle money can be rented out: that is, deposited into savings or an investment account.

Failure to do so actually depreciates the money because of inflation. Think of inflation as: "well, you worked for me for an hour last week, so I'll give you an hour's worth of goods this week, but when you worked for me five years ago for an hour, I can barely remember that, so you get a half hour's worth of goods today". Thus, no sensible person has idle money: it's all "in" something.